Method and plant for separating soap from black liquor using a pressure screen

ABSTRACT

An alkaline cellulose pulp pulping process treats soap and fiber-containing liquids to separate fibers from the liquids prior to further treatment of the liquids (e.g. by evaporation and combustion, or by cooking to produce tall oil). Black liquor containing soap, and/or washer filtrate containing soap, are fed to one or more soap separating tanks, and then the soap fraction from the top of the tanks is fed to a pressure screen where fiber separation takes place (preferably at a temperature of at least 70° C.). Black liquor may be added to the soap fraction to dilute it, and preferably simultaneously raise its temperature. The pressure screen has a screen surface with holes of 0.4 mm diameter or less, or slots with a width of 0.2 mm or less.

CROSS REFERENCE TO RELATED APPLICATION

This is a U.S. national phase of PCT/FI97/00273.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a method and apparatus for intensifyingan alkaline pulping process according to which at least soap-containingspent liquors from digestion and soap-containing washing filtrates frombrown stock washing are recirculated in the digester house, the brownstock washing plant or between these. The invention relates inparticular to treatment of at least the soap fraction separated fromliquors and filtrates in order to separate fiber material therefrom.

In a chemical alkaline pulping process, lignocellulose-containingmaterial, such as wood chips, is treated by digesting it in a solutionthe active chemicals of which are mainly sodium sulphide and sodiumhydroxide, or sodium hydroxide alone. After digestion, spent liquor isseparated from the pulp which is carried to further treatment such aswashing, screening and bleaching. Chemicals are recovered from the spentliquor, i.e. black liquor, for production of fresh cooking solution.

During the digestion process, greases and resin acids of wood saponifyand form sodium soaps, sulphate soap. Conventionally, soap is separatedfrom black liquor prior to feeding the liquor to evaporation andcombustion to form green liquor and further white liquor. As soap islighter than liquor it rises to the surface and is readily decanted off.It is important to remove soap as fully as possible because it is astrong scummy substance and thus causes many problems in the process.However, it should be noted that part of the soap has been dissolvedinto the black liquor and thus it is not separated to the surface of theliquor.

In addition to the digestion, soap is transferred from the pulp to theliquor also during washing of the pulp. The solubility of soap isdifferent from that of other dry solids and thus it may not be as easilywashed as other dry solids. In filter washing, soap is removed from thepulp at a later stage than other dry solid. Foaming soap also hampersfilter washing of pulp.

As known, chemical pulp may be produced by an alkaline cooking processboth in batch digesters and in continuous digesters. Instead of alwaysfeeding the black liquor from the digester to recovery of chemicals, hotblack liquor may be used to pretreat wood chips before the digestionstage itself in order to improve the process. In a continuous digestion,hot black liquor may be recirculated to pretreat wood chips in theimpregnation stage as has been described for example in EP patent no.527 294. Hot, pressurized black liquor may also be flashed to producesteam and to raise the dry solids content of the black liquor. In batchdigestion, black liquor may be stored at different temperatures inliquor tanks, "liquor batteries", for use in a pretreatment of chips.

Recirculation of black liquor provides several advantages. For example,the heat economy of the process is improved and, when sulphur is presentin the digestion, black liquor augments the volume of sulphur compoundsin the cellulose pulp which in the impregnation stage have a veryfavourable influence of decreasing the Kappa number of the pulp.Recirculation of black liquor brings about problems, also, as the soapcontained in the black liquor accumulates in the process if it is notefficiently removed from the solutions recirculated.

Efficient removal of soap in the processes described above has not beenpaid necessary attention to before. EP patent application no. 520 452discloses a method having three tanks for black liquor. At the beginningof displacement with washing filtrate, which takes place afterdigestion, black liquor is introduced into a first hot liquor batteryessentially at the cooking temperature and pressure and at the drysolids content reached during the digestion. Black liquor, thetemperature of which corresponds to the boiling temperature of liquor inan atmospheric pressure, is fed into a second black liquor battery. Thedry solids content of the black liquor in the second battery is lowerthan that of the black liquor in the first battery because the blackliquor mentioned first contains remarkable amounts of wash liquid andconsequently also its soap content is higher. According to this EPpublication, the second black liquor is used to heat the white liquor tobe introduced into the digester and after that the black liquor istransferred to a tank in which soap is separated from it. Subsequently,the black liquor having a temperature of less than 100° C. is used toimpregnate the chips of a new digestion batch. The chips are impregnatedafter that also with the hot black liquor from the first tank.

It has been noticed recently, however, that the measures of removingsoap described above are not adequate. If soap is not removedefficiently enough from liquids reused, such as black liquor and washingliquids, soap will accumulate in the pulp in the digester and willdissolve from it during further treatment. When pulp is further removedfrom the digester to a blow tank and a brown stock washing, soap causingstrong froth formation gives rise to problems in the blow tank and washfilters and thus causes production disturbances and decreases the pulpquality.

As described above, wood chips are conventionally treated in batchdigestion today with black liquor before digestion of the chips withwhite liquor. Grounds for this are for example the advantages providedby the black liquor treatment, for example an improved energy economy.Naturally, this presupposes optimal operation of the process. Removal ofsoap from the solutions containing soap helps in reaching this goal. Thesoap content of the black liquor in a batch digestion depends on thespot of the displacement process in the digester from which the liquoris recovered. As described above, black liquor is discharged from thedigester during the displacement at least to two tanks. The soap contentof the black liquor increases when the wash filtrate introduced into thedisplacement process from brown stock washers is discharged from thedigester. In Scandinavian circumstances, the soap content of pulpentering a brown stock washing department may be 80-100 kg/t_(m). Byregulating the soap content of the wash filtrate to be brought to thedisplacement, the soap volume ending up in the digester may be reducedas black liquor containing brown stock wash filtrate is typically usedin the impregnation of the next digestion batch. Further, it isadvantageous to remove soap from this black liquor before thisimpregnation. Also the soap content of the black liquor removed at thebeginning of the displacement process may be reduced if desirable. Allthese measures may be used to reduce the soap amount entering the blowtanks and further the brown stock washing department with the pulp.

In continuous digestion, black liquor is recirculated to pretreatment,sludging and impregnation of chips. Black liquor is removed from thedigester and, if desired, transferred to flash tanks before it isreused. The potential problems caused by soap may be avoided bycontrolling after the flash tanks the soap content of the black liquorto be used for pretreatment of chips.

When more efficient soap separation is pursued it should be kept in mindthat fibers are separated both into black liquor and into wash filtrateswhich hamper evaporation of black liquor by fouling evaporator surfacesand thus reduce the efficiency of the process and in the worst case eveninterrupt the production. For this reason, prior art methods tend toseparate fiber fraction from black liquor before evaporation thereof bya drum filter in which fiber fraction remains onto a wire surface of afilter drum while black liquor is filtered through a fiber layer and thewire surface and is pumped onward for further treatment, for exampleevaporation. A process of this kind has been disclosed for example inthe PCT application no. WO 9303815. A drawback of these processes isthat only a part of the black liquor is guided to recovery of chemicalswhile a large portion is returned for reuse for example in theimpregnation of chips. Thus, it is quite unnecessary to separate fiberfraction from a liquor which in any case is returned to the digestionprocess.

Just as black liquor, also the soap fraction separated/to be separatedfrom the liquors of a digestion plant and a wash plant contains fibers.Until now, fibers have not been separated from the soap fraction becausemill experience has shown that a filter drum used in the filtering offibers from black liquor is not applicable for this purpose. Thecomposition of this fiber fraction is significantly different from thatof black liquor. The soap fraction is a fluffy lumpy material separatedfrom the surface of liquor for example in a separation tank as anoverflow. If this fluffy lumpy fraction is distributed onto a drumfilter in which a fiber mat is gradually formed onto a wire, it is clearthat lumpy soap remains on the same side of the wire surface as thefiber mat and consequently only black liquor becomes separated from thefibers and soap. The separation of fibers from the soap fraction isfurther hampered by the fact that fibers travelling with the soapfraction are mostly attached to the soap lumps. When this kind of soapfraction is carried without separation of fibers to an evaporation plantthe fiber fraction impedes the operation of the evaporator. Further, ifthe soap is used in the production of tall oil, the fiber fractionhampers the tall oil cooking process.

In U.S. Pat. No. 3,109,839 a process for production of organic acids hasbeen diclosed. The process uses alkali soap skimmings from sulphate pulpmanufacture as raw material for organic acids. The process includestreating the soap skimmings with both dispersing agents and mineralacids to decompose alkali metal soaps and to form organic acids andalkali metal and alkaline earth salts. After the decomposition of thesoaps the reaction mass is transferred to a screen for separating fibersfrom the reaction mass. The process avoids the blockage of screensurfaces by means of breaking the soap molecules into smaller moleculeswhich do not interfere with the fiber separation.

An object of the present invention is, among other things, to eliminatethe problem mentioned and to provide a method of reducing the fibercontent of soap-containing solutions discharged from a pulping processso that fibers do not cause problems in further treatment of the soapfraction or a combined fraction of soap and black liquor.

The characteristic features of the present invention developed forexample to reach the object mentioned above are disclosed by theappended patent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically a pressure screen used in a preferredembodiment of the method of the invention; and

FIG. 2 illustrates schematically a preferred embodiment of the method ofthe invention applied in a batch digestion process.

DETAILED DESCRIPTION OF THE DRAWINGS

In the process of FIG. 2, illustrating a process according to apreferred embodiment of the invention, a batch digester 10 has beencharged with wood chips and white liquor supplied in lines 41 and 42from a tank 40, for digestion. When the digestion performed at a raisedtemperature, at about 155-180° C., has been completed, black liquorproduced during the digestion is discharged from the digester bydisplacing it with wash filtrate 54 obtained from brown stock washing53. The hot black liquor discharged first is taken in line 21 to a hotliquor battery. The temperature in the hot liquor battery is almost thesame as the cooking temperature.

The temperature of black liquor discharged from the digester 10 at acertain stage of the displacement process decreases as the temperatureof the displacement liquor, i.e. of the wash filtrate 54, is less than100° C. This liquor having a lower temperature is taken in line 31 to asecond liquor battery 30. After the displacement, pulp is discharged inline 11 to a blow tank 50.

The black liquor in the batteries 20 and 30 may be reused to pretreatchips for the next digestion batch. The black liquor in the battery 30having the lower temperature contains soap from brown stock washfiltrate and pulp and the soap has to be removed as completely aspossible before the liquor is guided to heat the next chip batch in thedigester 10. In practise this means that the amount of soap to beremoved is the amount entrained in the black liquor from the washfiltrate and pulp. According to the invention, the liquor is guided forthis purpose from the battery 30 in line 32 to a soap separating tank33. The tank 33 has been dimensioned so as to provide an adequateretention time to allow soap, being a lighter substance than liquor, tobe separated to the surface of the liquor. Soap may be removed by knownsoap separation methods by means of which soap can be skimmed off fromthe surface of the solution. The tank 33 may for example be providedwith an over-flow for the soap fraction. Depending on the mill, soap isremoved from the soap separation tank in line 35 for example either totall oil production or evaporation and combustion. Essentially soap-freeblack liquor is taken in line 36 to the digester 10 to heat andimpregnate chips. The black liquor supplied to the tank 33 contains alsofibers which are not separated to any particular fraction but part ofthem remain in the soap-free black liquor and are drifted back to thedigestion and part of them are transported further entrained in the soapfraction.

The heating and impregnation black liquor is displaced from the digester10 by black liquor from the battery 20 and is discharged from thedigester in line 22 to the tank 33 for separation of soap and farther inline 37 to a black liquor evaporation plant. As the black liquor in line37 is essentially soap-free the line 37 may be provided with for examplea cylinder-type filter 34 to separate fibers from the black liquor.

According to the invention, the line 35 discharging fiber-containingsoap fraction from the soap separation tank for example either to talloil production or evaporation and combustion has been provided with apump and a pressure screen 38 with which soap fraction is treated sothat the screen rejects fibers in the soap and accepts soap itself andblack liquor with it. In other words, fiber fraction is meant to becollected onto the screen surface and be removed therefrom for reusewhile soap fraction passes through the fine apertures in the screensurface and is discharged from the apparatus in a line 39 for furthertreatment of soap. The pressure screen 38 (FIG. 1) used typicallycomprises a pressure-tight housing 2 and connections therein for thematerial to be fed in, reference number 4, for the fraction passedthrough the screen drum, i.e. the accept 6, and for the fraction whichhas remained on the surface of the screen drum, i.e. the reject 8. Thehousing surrounds a combination of a screen drum and a pulse member inwhich either the screen drum or the pulse member is rotating.

Usually the pulse member is a so-called rotor, i.e. a revolving meansgenerating a turbulence field in the vicinity of the screen drum surfacewhich on one hand prevents the screen drum apertures from being cloggedand on the other hand, during normal operation of the screen, breaks upfiber lumps in the vicinity of the screen drum surface. For example U.S.Pat. No. 5,000,842 describes a typical pressure screen. In this type ofuse, the apertures in the screen drum of the screen 38 must be verysmall in order to reject fibers. When holes are used the diameter of theholes must be at the most 1.0 mm, preferably less than 0.5 mm, mostpreferably about 0.20-0.25 mm. When a slotted drum is used the width ofthe slots must preferably be at the most 0.20 mm. When the apparatusemploys also a rotor generating a relatively strong turbulence,practical experience has proved that the turbulence breaks up soap lumpsso that also fibers are detached from them and consequently soap, beinga finer substance, passes through the apertures of the screen surfaceand fibers remain on the screen surface and are transported along thesurface to the discharged opening of the screen.

A method according to a preferred embodiment of the present invention ischaracterized in that soap fraction is diluted with black liquor so thatthe share of the soap fraction is preferably less than 50% of the totaldry-solids volume to be screened. Preferably the dilution is carried outwith hot black liquor so that the temperature of the suspension in thescreening process is higher than 60° C., preferably higher than 70° C.

When fiber fraction has been separated from both the essentiallysoap-free black liquor and the soap fraction, the soap and the blackliquor may be combined and transported for further treatment.

Before hot black liquor from the battery 20 is guided to a digester, italso may be treated in a corresponding way as the cooler black liquorfrom the battery 30 in order to remove soap from it. The hot blackliquor flowing in line 23 may at first be used to heat white liquor 44in a heat exchanger 45. Soap is separated from the solution in a tank25. Soap is discharged in line 26 and the hot black liquor having a lowsoap-content is pumped in lines 27, 42 to the digester to displace acooler black liquor.

Quite in the way described above, fiber fraction may be separated in apressure screen 28 also from a soap fraction obtained from the soapseparation tank 25. Further, soap fractions from both the separationtanks 33 and 25 may be combined and treated in one and the same pressurescreen in order to separate fibers from the soap fraction.

Following the digestion, pulp is supplied after a blow tank 50, astorage tank 51 and a potential knot separation 52 to a washing plant tobrown stock wash filters 53. Large amounts of soap may be transferredfrom the pulp to the wash filtrate in the washers. When such washfiltrate is recirculated to a digester to be used as displacementliquid, soap will consequently accumulate in the pulp and soap, foamingheavily, will cause problems in the blow tank. Accumulation of soap maybe prevented at this stage of the process by providing the process witha soap-separating tank 56 for treatment of the wash filtrate, theseparated soap being discharged from the tank in line 57. Also this soapfraction may be treated either with a pressure screen of its own, or itbe combined with the soap fractions described above (FIG. 2 illustratesan alternative of combining the soap fractions from displacement tanks56 and 25 and treating them together in a pressure screen 28) wherebythe process may manage with a minimum of one pressure screen forseparating fibers from different soap fractions.

EXAMPLE

In a Finnish pulp mill, a pressure screen was connected to a deviceillustrated by reference numeral 38 in the figure. The operation of thepressure screen was monitored for several weeks by determining the fibercontent (mg/l) in the feed, the accept and the reject of the pressurescreen and the soap content (%) in the feed and reject of the pressurescreen. The test in the first period was performed by using a screendrum with a hole diameter of 0.2 mm. It was proved that the meanseparation efficiency during the test period was about 64%. The test inthe second period was performed using a screen drum with a hole diameterof 0.4 mm. It was found out that the separation capacity was essentiallypoorer, i.e. about 14%. When the separation efficiency was tested usinga higher fiber contents in the feed flow it could be stated that even adrum having "large holes" can reach a good separation efficiency, about83%, when the fiber amount in the soap fraction is high. Further, thetest proved that the soap content of the reject is about 70-75% of thesoap content of the feed flow. This indicates that returning the fiberreject for reuse does not cause accumulation of soap in the returncirculation but the apparatus works as it was expected to work.

As may be understood from the above, a method of a new type eliminatingthe drawbacks of prior art methods has been developed for improving thechemical recovery circulation of a pulping process, the method beingmainly based on treating soap fractions from different partial processesas much without problems as possible. It should also be remembered thatalthough the invention has been applied to a batch process as analternative, the method according to the invention is in a quitecorresponding way applicable to a continuous process in connection withwhich, too, the problems caused by soap and the fiber content of soapare known and admitted. Thus, according to the invention, soap may beseparated from one or several soap-containing solutions of a continuousdigestion process, or different soap-containing solutions may becombined and the soap-containing fraction may be removed from them andtreated further in a pressure screen in order to separate fibers fromthe soap-containing fraction. Further, it is quite possible according tothe invention to separate soap from soap-containing solutions by usingsome other means than the soap-separating tank described above. Thus,the method according to the invention described above may be applied toany other point of the pulp manufacturing process treatingsoap-containing fractions, for example a batch or a continuousdigestion, a washing plant or an evaporation plant. Further, the soapfractions from different pulp production departments may be combined ina desired way and treated further in one or several pressure screens inorder to separate fibers from the soap fraction.

We claim:
 1. A method of treating an alkaline pulping process soap andfiber containing liquid downstream of a soap separator comprising: (a)separating fibers in the soap and fiber containing liquid from theliquid in a pressure screen to produce a soap-containing liquid havingreduced fiber content to facilitate and intensify further treatment ofthe soap-containing liquid.
 2. A method as recited in claim 1 wherein(a) is practiced using soap-containing black liquor directly from analkaline digestion process as the soap and fiber containing liquid.
 3. Amethod as recited in claim 1 wherein (a) is practiced usingsoap-containing black liquor from a digestion process as the soap andfiber containing liquid and further comprising (b) diluting thesoap-containing liquid from an alkaline digestion process with otherblack liquor prior to practicing (a).
 4. A method as recited in claim 3wherein (b) is practiced to simultaneously raise the temperature of thesoap-containing black liquor so that (a) is practiced on liquid at atemperature of at least 60° C.
 5. A method as recited in claim 4 wherein(b) is practiced to simultaneously raise the temperature of thesoap-containing black liquor so that (a) is practiced on liquid at atemperature of at least 70° C.
 6. A method as recited in claim 4 wherein(b) is further practiced so that the soap portion is less than 50% byvalue of the total dry-solids volume of the liquid treated in (a).
 7. Amethod as recited in claim 1 wherein (a) is practiced with the liquid ata temperature of at least 60° C.
 8. A method as recited in claim 1wherein (a) is practiced using brown stock wash filtrate as the soap andfiber containing liquid.
 9. A method as recited in claim 1 wherein (a)is practiced using a combined stream of soap-containing black liquorfrom an alkaline digestion process and brown stock wash filtrate as thesoap and fiber containing liquid.
 10. A method as recited in claim 1wherein (a) is practiced by screening in a pressure screen having ascreen surface with holes 0.4 mm or less in diameter or slots 0.2 mm orless in width, to produce a soap-containing liquid having reduced fibercontent.
 11. A method as recited in claim 10 wherein (a) is furtherpracticed so that soap passes through the holes or apertures and fiberspass along the screen surface.
 12. A method as recited in claim 10wherein (a) is practiced so that the soap content of rejects from thescreen is about 70-75% of the soap content of the soap-containing liquidfed to the screen.
 13. A method as recited in claim 1 wherein furthercomprising, after (a), (b) further treating the soap-containing liquidby evaporation and combustion, or tall oil production.
 14. A method asrecited in claim 1 further comprising, prior to (a), (b) passing blackliquor from a continuous or batch alkaline pulping process to aseparation tank to produce a soap fraction at the top of the tank, andpracticing (a) using the soap fraction from (b); and wherein (a) ispracticed using a pressure screen having a screen surface.
 15. A methodas recited in claim 14 wherein (a) is practiced by screening in apressure screen having a screen surface with holes 0.4 mm or less indiameter or slots 0.2 mm or less in width, to produce a soap-containingreject liquid having reduced fiber content.
 16. Apparatus forintensifying an alkaline cellulose pulping process, comprising:adigester; pulp washers operatively connected to said digester which washpulp from the digester; at least one soap separator which separates soapfrom soap and fiber-containing liquid from at least one of saiddigesters and said washers to produce a soap fraction containing fibers;a discharge line from said soap separator, containing the soap fractionwith fibers; and a pressure screen connected to said discharge line toseparate fibers from soap containing liquid.
 17. Apparatus as recited inclaim 16 wherein said soap separators comprise a plurality of soapseparating tanks; and further comprising a pump connected between atleast one of said tanks and said pressure screen.
 18. Apparatus asrecited in claim 17 wherein at least two of said tanks have dischargelines connected to said pressure screen so that said pressure screentreats soap fractions from said at least two tanks at the same time. 19.Apparatus as recited in claim 16 wherein said pressure screen comprisesa screen surface having holes with a diameter of 0.4 mm or less. 20.Apparatus as recited in claim 16 wherein said pressure screen comprisesa screen surface having slots with a width of 0.2 mm or less.